Nitriding of zirconium cathode for arc-heater testing in air

Zirconium cathodes with a nitrided surface layer are promising candidates for arc-heater tests in air because of their good erosion resistance and excellent physical properties. Nitriding of a zirconium cathode is performed by a microwave plasma generator. Nitrided cathodes yield a stable plasma flow of argon–air up to 80 A at a maximum flow rate of 4 l/min argon and 0.8 l/min air for a long duration. The specific input power corresponds to 11.8 MJ/kg. During arc-heater testing, nitriding is a governing reaction and golden zirconium nitride is formed at the tip of the cathode surface. The formation mechanism of a ceramic layer during microwave plasma generator treatment and arc-heater testing will also be discussed.